StringCount

StringCount["string", "sub"] counts occurrences of a substring.

StringCount["string", patt] counts substrings matching a pattern.

Examples

Count occurrences:

StringCount["mississippi", "ss"]
(* 2 *)

Count pattern matches:

StringCount["abc123def456", DigitCharacter]
(* 6 *)

Please visit the official Wolfram Language Reference for more details.